1. Field of the Invention
The present invention relates to a photothermographic material. More particularly, the invention relates to a photothermographic material which exhibits improved image storability and improved resistance to image defects.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis and in the field of films for graphic arts, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. For this reason, technology regarding thermal developing image recording materials for medical diagnosis and for graphic arts, which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, is required. The thermal developing image recording materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image recording materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described, for example, in the specifications of U.S. Pat. Nos. 3,152,904 and 3,457,075 and in “Thermally Processed Silver Systems” by D. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein.
In particular, photothermographic materials generally have an image forming layer in which a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region.
Most conventional photothermographic materials have been generally produced by a process using an organic solvent such as toluene, methyl ethyl ketone, or methanol as a solvent. However, the use of an organic solvent as the solvent is not advantageous, not only in view of undesired effects on the human body during the manufacturing process, but also in view of the cost due to recovery of used solvents and other factors.
In view of the above, a manufacturing method using an aqueous dispersion of a hydrophobic polymer as a binder has been disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-10670 as a method of manufacturing a photothermographic material using a coating solution of an aqueous medium which does not require such concern. Further, JP-A No. 11-84573 discloses the use of a specified polymer latex as a binder of a surface protective layer.
These photothermographic materials utilizing an organic silver salt have a great characteristic of containing all components necessary for image formation in the film in advance and being capable of forming images only by heating. However, there is a problem in that it is difficult to adjust the color tone of a developed silver image to be favorable. Furthermore, because these components remain as unreacted components or reaction products after image formation, there is another problem in that these remaining components and reaction products exert adverse influences on storage stability of the image.
Further, in the manufacturing method using the aqueous dispersion of the hydrophobic polymer described above as a binder, since a styrene-butadiene polymer which is a raw material for synthetic rubbers is used as a polymer for use in the binder, the film quality is soft, and improvement has been demanded with respect to forces such as pressure, scratching, and the like.